This application relates to a heat pump refrigerant system, wherein the compressor is a two-stage compressor, and wherein an intercooler is provided between the two compression stages. The intercooler is preferably subjected to the ambient airflow and is placed upstream of an outdoor heat exchanger with, respect to this ambient airflow, such that the cooling in the intercooler is preferably provided by the circuitry and components that are already part of the refrigerant system.
Heat pumps are known in the air conditioning art, and are utilized to provide both heating and cooling of a secondary fluid, such as air, delivered into an environment to be conditioned. A typical heat pump includes a compressor, an expansion device, an outdoor heat exchanger and an indoor heat exchanger. Typically, a four-way valve reverses the flow of refrigerant throughout the system between a cooling and heating mode of operation. The refrigerant flows from the compressor to the outdoor heat exchanger when the refrigerant system is in a cooling mode, and from the compressor to the indoor heat exchanger when the refrigerant system is in a heating mode.
To obtain additional capacity, enhance system efficiency and achieve higher compression ratios, it is often the case that a two-stage compressor is provided in a refrigerant system. With a two-stage compressor, two separate compression members or two separate compressor units are disposed in series in a refrigerant system. Specifically, for instance, in case of a reciprocating compressor, two separate compression members may be represented by different banks of cylinders connected in series. Refrigerant compressed by a lower stage to an intermediate pressure is delivered from a discharge outlet of this lower stage to the suction inlet of the higher stage. If the compression ratio for the compressor system is high (which is typically the case for two-stage compression systems) and/or refrigerant suction temperature is high (which is often the case for a refrigerant system equipped with liquid-suction heat exchanger), then refrigerant discharge temperature can also become extremely high, and in many cases may exceed the limit defined by the safety or reliability considerations. Thus, it is known in the art to provide an intercooler heat exchanger (or a so-called intercooler) between the two compression stages to extend the operational envelope and/or improve system reliability. In an intercooler, refrigerant flowing between the two compression stages is typically cooled by a secondary fluid. Quite often, additional components and circuitry are required to provide cooling of the refrigerant in the intercooler. As an example, a fan or pump is supplied to move a secondary cooling fluid from a cold temperature source to cool the refrigerant in the intercooler.
Recently, new generation refrigerants, such as natural refrigerants, are being utilized in refrigerant systems. One very promising refrigerant is carbon dioxide (also known as CO2 or R744). However, particularly with the CO2 refrigerant systems, the intercooler becomes even more important as these systems tend to operate at high discharge temperatures due to high operating pressures, frequent use of liquid-suction heat exchanger, and, in general, by the transcritical nature of the CO2 cycle, as well as a high value of the polytropic compression exponent for the CO2 refrigerant. However, the additional cost of the circuitry and components associated with the intercooler, along with the limited benefits for the prior art refrigerant systems utilizing conventional refrigerants, made the provision of an intercooler in the conventional refrigerant systems less desirable.
Thus, it is desirable to provide an intercooler for a multi-stage compressor refrigerant system, and particularly for a CO2 heat pump refrigerant system, that essentially does not require any additional circuitry or components beyond the intercooler itself.